A known unit, for example from FIG. 1 of the document US 2011/0031401 is an optical detector unit that operates on front side illumination and in a range of infrared wave lengths, having an elementary infrared detector, or detecting element, formed in a HgCdTe (Mercury Cadmium Telluride) semiconductor structure and created in the form of an N—P junction photodiode. The infrared detector further also has a passivation layer located on both sides of the semiconductor structure. In operation, the infrared detector converts a flux of incident photons into an electrical signal.
According to a hybrid architecture, the optical detector unit also includes a read out circuit, also known as ROIC, the acronym for Read Out Integrated Circuit, assembled with the infra-red detector by means of an epoxy layer. The read out circuit, capable of processing the electrical signal emanating from the infrared detector has an electrode intended for receiving this electrical signal. This electrode is connected to the infra-red detector by means of an electrical contact formed in an electrical interconnection hole, also known as via, which passes through both the semiconductor structure as well as the dielectric passivation layer.
One advantage of this optical detector unit is that the establishment of the electrical contact in the electrical interconnection hole at the level of the infrared detector, and more specifically in contact with a doped zone N of the N—P junction photodiode, makes it possible to minimize the dark current generated in the photodiode.
However, this optical detector unit has a number of disadvantages.
In the first place, the operation of the optical detector unit is limited to the infrared spectrum. A known technique in order to extend the operation in the visible wavelengths of infrared detector units is to modify the structure of the infrared detector, for example by removing the passivation layer located on the semi-conductor structure of the infrared detector. The problem is that this technique leads to changes in contrast with respect to the image generated at the output of the optical detector unit, which creates difficulties in interpretation and identification on this image.
In the second place, the resolution of an optical detector unit of the aforementioned type including a plurality of elementary detectors, for example, in the form of a matrix array, is limited. A conventional technique for increasing the resolution, that is to say, the number of elementary detectors, on a given surface is to reduce the width of the elementary detectors. This reduction leads to the closure of the electrical contacts, the electrical interconnection holes, and the N—P junctions between each elementary detector, which is a generator of cross talk between detectors. Also, this problem typically limits the width of an elementary detector to ten microns.
On the other hand, another drawback of the presence of the electrical interconnection hole opening on to the surface of the elementary detector, is that it limits the rate of filling of the elementary detector and therefore the sensitivity of the optical detector unit, with this fill rate being defined as the ratio between the surface of the elementary detector used for the detection of light and the total surface area of the elementary detector.
One object of the invention is therefore to provide an optical detector unit with a wide spectral range of operation, that is capable of multispectral detection, while at the same time offering improved performance as compared to the aforementioned state of the art.